The Tomb of Sargeras (formerly Temple of Elune) was used by Aegwynn after she "defeated" Sargeras, (later turning out that it was just his avatar). She sealed Sargeras' avatar inside the tomb and guarded it with the Maiden of Vigilance. But when the Legion came, they seek to unleash the avatar once again and erase what Aegwynn did into history.

As is usually the case with new raids, the Tomb of Sargeras is being released gradually, between June and August of 2017. The official release schedule is as follows (starting a week after Patch 7.2.5 dropped):[1]

The vast majority of creatures encountered within the Tomb are demons. Deeper down, members of the naga race can be found. What remains of the Avatar of Sargeras is encountered towards the end of the encounter.